1. Field of the Invention
This invention relates generally to distortion canceling techniques and more specifically relates to feed forward circuits and techniques for altering the amount of signal noise and distortion provided by amplifying devices.
2. Description of the Prior Art
Amplifiers are customarily used for enhancing electrical signals. However, a common problem associated with such signal enhancement is the addition of amplifier noise and distortion to the electrical signal. This is especially true when power amplifiers are utilized. Traditionally, feed forward circuits and techniques have been used to reduce the amount of signal noise and distortion generated by such amplifying devices.
It is well known that most applications that involve radiating power (through an antenna of some sort) require some degree of power amplification in order to overcome the inevitable space loss. It is also known that at higher power these power amplifiers are more likely to distort the signal in some way. Most of the modern transmission signals have limitations on how much distortion is permissible. Achieving power amplification which is cost effective, efficient (i.e., does not require a lot of external power), small sized, and achieves adequate linearity has become a challenge to modern designers of these devices.
There are many ways that the linearity requirements of a power amplifier are specified. Typical ones involve specifying the level of spectral distortion that can be tolerated (i.e., the amount of spurious signals outside the allocated bandwidth of the signal) or specifying the accuracy in vector space (i.e., monitoring the locus of the output voltage in amplitude and phase in vector space) in some fashion. Conventional feed forward power amplifiers utilize delay lines that typically are bulky and cause unneeded circuit loss.
In addition, feed forward power amplifiers are typically built as an assembly of modules that are mounted on a chassis or housing and are interconnected. Often exotic and costly substrates such as glass or ceramic loaded Teflon™ are utilized. It would be desirable to integrate a large portion, if not all, of a feed forward amplifier on a semiconductor substrate. Digital signal processing (DSP) offers a way of providing such an integration. DSP converts an analog signal to a digital signal, processes it, then may convert it back to an analog form. A limiting factor in DSP has been the speed of analog to digital conversion, and digital to analog conversion; however, recent technology has overcome these problems.
A signal being DSP processed can take advantage of a low-cost circuit typically utilizing digital gates, on a low cost substrate such as CMOS (Complementary Metal Oxide Semiconductor), to perform the same function previously done in analog form. Alternatively, a DSP may be implemented by a program running on a processor circuit. As a result of utilizing one of these techniques, the implementation will be smaller, more reliable and lower in cost. The present invention combines elements of feed forward amplifiers, digital pre-distortion amplifiers, and DSP techniques.
FIG. 1 is a block diagram of a conventional feed forward power amplifier. An example of two carriers (the desired signals) in the frequency domain is shown as they progress through the system. A portion of the input signal is coupled using a directional coupler 2 through the main amplifier path where its amplitude and phase are adjusted (Block 4) prior to amplification in the main power amplifier 6. The output of the main amplifier 6 is a greatly amplified version of the input signal and, in the process, is distorted. Shown in FIG. 1 are the third order intermodulation distortion (IMD) products around the two carrier signals. These are unwanted spurious signals.
In another path from the input, the unaltered input signal is delayed (Block 8) enough so as to match the delay through the main amplifier path. Then, in a coupler 10 or the like, the input signal is combined with a sample of the amplified input signal (taken with coupler 11) which is adjusted to have the same amplitude but is 180° out of phase such that the desired carrier signals cancel out. In practice, this is difficult to accomplish completely, but typical systems achieve 30 to 40 dB (decibels) of cancellation. What is expected to remain are the error signals which are the unwanted signal components (i.e., noise and distortion, including spurious signals). These are amplified (Block 12) in the correct phase and amplitude, and combined using a combiner 14 or the like out of phase with the signals from the main amplifier 6 which have been delayed (Block 16) to account for the delay in the error amplification process. The resultant output signal consists of the amplified desired signals and the unwanted signals which have been reduced by typically 20 –30 dB in the combining process.
FIGS. 2a and 2b are graphs which help explain how feed forward techniques achieve linearization. More specifically, FIG. 2a is a graph of the transfer curve of the main amplifier 6, with Vout, i.e., voltage out of a power amplifier with or without a pre-distorter circuit, as will be explained, as the ordinate, and Vin, i.e., voltage in to the power amplifier, with or without a pre-distortion circuit, as the abscissa. Plotted in the graph is the desired transfer curve, shown as a dashed line, the power amplifier actual transfer curve, and the output of the error amplifier 12 coupled onto the output of the power amplifier. FIG. 2b is a graph of superimposed signal spectrums, in the frequency domain, showing the desired signals, the intermodulation (IM) products on the output of the main amplifier 6, the ideal output of the error amplifier, and the ideal output of a feed forward amplifier when the error amplifier output is summed with the main amplifier output. These two figures are another way of viewing the feed forward process.
In FIG. 2a, the amplitude transfer function is shown, which typically compresses as the input signal drives the main amplifier closer to its saturation output power level where it is unable to develop any more power. The feed forward amplifier compensates for the lack of output power in the main amplifier 6 by adding in the difference between the desired linear output and the actual output from the main amplifier. This can be accomplished until the error amplifier 12 is unable to deliver enough difference power.
Typical feed forward amplifiers are able to amplify multiple signals with IM (intermodulation) levels down approximately 70 dBc. There can be, however, a number of issues with this approach.
First, because the high power output of the main amplifier 6 must be delayed, there is loss in the delay element. Low cost delay elements are delay lines which have higher loss than delay filters but the latter are expensive. Either delay element is also large to keep the loss low. Typical losses are 0.5 dB which means that ten percent (10%) of the power is lost.
Second, because the cancellation process is approximate, the amplified error signal contains a considerable amount of desired signal. Typically, the majority of the power in the error signal is in the desired signals. Thus, the error amplifier 12 must be large enough to amplify the undesired signals to levels high enough to cancel those in the main output signal but still have enough power capability not to saturate due to the amplified desired signals. As a result, the error amplifier 12 must be quite large and utilizes a lot of power, thus further reducing the efficiency of the overall amplifier.
Third, the ultimate efficiency of a feed forward amplifier is limited, based on the first and second reasons set forth above.
Fourth, in order to achieve the amount of required cancellation, the phase and gain and group delays of the two loops in a feed forward amplifier must be tightly controlled. Typical systems use fixed nominal values of delay, and control the phase and gain (Blocks 4 and 17), as shown in FIG. 2, with the monitoring circuits 18 and 20 and their associated couplers 22, 24, the first of which samples the unamplified error signal and the second of which samples the feed forward amplifier output signal. There are believed to be many patents covering techniques for controlling these loops. These correspond to various ways to inject “pilot” tones (inserted signals with known characteristics which are then cancelled in the loops), ways of adding modulation to the desired signal, ways of correlating the output with the desired signal, and the like. Most of these require some level of assumption on the stability of the system. For instance, a typical feed forward amplifier system “assumes” that the gain and phase linearity of the power amplifiers is reasonably controlled.
Fifth, as a result of the basic architecture and components required, feed forward amplifiers tend to be complex which affects their reliability and cost.
FIG. 3a shows a block diagram of a conventional digitally pre-distorted amplifier 28. Also shown in FIG. 3b is the transfer curve of the pre-distorter and the amplifier. These systems operate by inverting the transfer curve of the power amplifier such that the amplified output of the pre-distorter, when amplified by the power amplifier, develops the desired output signal. As shown, as the power amplifier gets closer to saturation, the output of the pre-distorter must “overcompensate” more and more to get the desired signal. Of course, there is no mechanism to develop more output power than can be delivered by the power amplifier output stage. By contrast, the feed forward amplifier can develop more output power than is available from the main amplifier since additional power is available from the error amplifier.
As shown, the digital pre-distorted amplifier operates by using a fixed algorithm to pre-distort the input signal. This algorithm is validated in an adaptive feedback loop, where a coupler 30 is used to sample the output signal of the amplifier 28, which signal is provided to an analog to digital converter 32, and the digitized sample signal is provided to the pre-distortion circuit 34. Thus, the adaptive feedback loop monitors the output of the amplifier 28 and makes non-real time updates to the algorithm. Various techniques are available for implementing the complex pre-distortion, but a typical one would involve a lookup table which monitors the input signal level and uses a lookup table to vary the output level and phase to achieve the desired inverse transfer characteristic of the power amplifier. The feedback system monitors a segment of the output signal and determines if the inverse curve is effective. If not, then the lookup table is varied somewhat to improve the match. Most digital pre-distortion systems characterize the AM-AM (amplitude modulation) and the AM-PM (phase modulation) characteristic of the amplifier. This recognizes that the major cause of distortion is the instantaneous amplitude of the input signal. Such causes the gain and the phase shift in the amplifier to vary (ideally, they would be constant over the input dynamic range). Another issue is “memory effect” which is so labeled because the status of the amplitude prior to the signal being amplified affects both the phase shift and the gain of the amplifier. A number of different causes for memory effect have been identified and a proper digital pre-distortion system recognizes the effect and attempts to compensate for it.
Typical digital pre-distorted power amplifiers do not achieve the level of performance of feed forward amplifiers. Typical numbers are IMD levels around −55 to −60 dBc. These amplifiers tend to have the following characteristics:
First, the efficiency tends to be higher than that achievable in a feed forward amplifier because they do not require the RF (radio frequency) output circuitry loss and the extra power dissipation in the error amplifiers.
Second, as noted, their level of spurious reduction is not as high as feed forward amplifiers. This is due to the difficulty in predicting the response of the power amplifier to all signals. Memory effect compensation tends to be accomplished by approximations, and the memory affect can vary with time, temperature, and frequency. A feed forward amplifier does not assume any distortion characteristics as it amplifies the error signals no matter what their source.